It has been known that calcium sulfate dihydrate upon heating in the dry state is first dehydrated to form beta-calcium sulfate hemihydrate, which upon further heating loses its water of crystallization and thereby is converted into calcium sulfate anhydrite. In this process, first the calcium sulfate anhydrite III (soluble calcium sulfate) is formed wherein the crystal structure of the calcium sulfate hemihydrate has mostly been retained. Thus, it can easily be rehydrated and undergo transformation into the calcium sulfate dihydrate in a per se known manner. Thus, calcium sulfate anhydrite III is a form of calcium sulfate capable of relatively rapid setting with water. Upon further heating, the calcium sulfate anhydrite II (insoluble calcium sulfate) is formed with a rhombic crystal lattice having closest packing and relative stability to water (at temperatures below 40.degree. C.).
From the European Published Unexamined Patent Application EP-OS 0,012,488, it is apparent that calcium sulfate hemihydrate is preferably formed in a suspension with sulfuric acid having a concentration of from 40 to 60% by weight at a temperature of from 40.degree. C. to 80.degree. C. in the presence of at least one tetravalent metal compound. From the European Patent Specification EP-PS 0,012,487, it is apparent that under comparable conditions, and more specifically in the presence of di-, tri- and/or pentavalent metal compounds, a calcium sulfate anhydrite having an average particle size of from 0.5 to 3.mu.m is formed. The processes described in the two European Patent Publications were preferably carried out with calcium sulfate dihydrate obtained in the course of the production of phosphoric acid. Thus, it is the so-called "phosphogypsum."
Further processes for the partial preparation of very finely divided calcium sulfate anhydrite, using very high temperatures, have been known, for example, from the U.S. Pat. No. 4,080,422 wherein ground dihydrate is blown at from 800.degree. C. to 1200.degree. C. into the hot gases of a flash-drier. According to the Japanese Published Unexamined Patent Application JP-OS 77-114494 (Chem. Abstracts 88, 1978, No. 12, No. 78089), phosphogypsum is calcined at 900.degree. C. with three to eight times the amount of calcium phosphate and ground, whereby an anhydrite II is formed which will again set when mixed with alum and water.
If calcium sulfate dihydrate and/or calcium sulfate hemihydrate is converted into anhydrite II by means of sulfuric acid and at low temperatures, it is always necessary to work in suspension and to employ relatively large amounts of sulfuric acid. Thus, for example, according to the European Published Unexamined Patent Application EP-OS 0 112 317, alpha-hemihydrate from a phosphoric acid wet process is heated in suspension in an excess of at least 35% sulfuric acid to at least 60.degree. C. for at least 60 minutes, whereby a finely divided anhydrite is formed. The particle size is between 2 and 40 .mu.m, depending on the process conditions. From the Norwegian Patent Specification 54 999 from the year 1931 as quoted therein, it is apparent that by heating calcium sulfate anhydrite III (soluble calcium sulfate) with 24% sulfuric acid, a finely divided amorphous anhydrite is formed. In said process the amount of acid has to be at least so large that the anhydrite is at least completely wet. Preferably, larger amounts of acids were employed. It has been noted that at higher acid concentrations and higher temperatures, considerable amounts of the calcium sulfate were decomposed to form calcium bisulfate or that upon use of a too diluted sulfuric acid, calcium sulfate dihydrate is formed.
From the U.S. Pat. Nos. 2,021,910, 2,822,242 and 2,956,859, it is also apparent that anhydrite is formed only in the presence of relatively high amounts of a relatively concentrated sulfuric acid and that higher temperatures greatly accelerate the conversion, however also result in the formation of more coarsely crystalline products.